With that gear ratio, 90/23 x 2.13= 8.33 FDR (2.13 is a sprint2's internal ratio), it shouldn't really be slow accelerating. Maybe there is some resistance in the drivetrain. Bad bearing, belts rubbing on the motor or the battery, etc. If you disconnect the motor(loosen the screws and drop the motor down so the pinion isn't touching the spur), does everything spin nice and freely? Does the motor get really hot, quickly.
You can try a smaller pinion. It should help improve acceleration, but I suspect there is some other issue going on here.
48p gears have larger teeth than 64p. 48p won't strip as easy as 64p, but doesn't allow for as fine adjustments to the gear ratio.